This invention relates to the manufacture of semiconductors, and more particularly to a method and apparatus for controlling the chemical-mechanical planarization (“CMP”) of semiconductor wafers in real time during the process, and particularly for determining when the end-point of the process has been reached.
As semiconductor devices are scaled down to submicron dimensions, planarization technology becomes increasingly important, both during the fabrication of the device and for the formation of multi-level interconnects and wiring. Chemical-mechanical planarization has recently emerged as a promising technique for achieving a high degree of planarization for submicron very large integrated circuit fabrication.
CMP is currently used for 0.35 μm device manufacturing and is generally viewed as a necessary technology for the manufacture of next generation 0.25 μm devices. Typically, CMP is used for removing a thickness of an oxide material which has been deposited onto a substrate, or on which a variety of integrated circuit devices have been formed. A particular problem that is encountered when a device surface is chemically-mechanically planarized/polished is the determination when the surface has been sufficiently planarized, or when the planarization end-point has been reached because when removing or planarizing an oxide layer it is desirable to remove the oxide only to the top of the various integrated circuit devices without, however, removing any portions of the latter.
In the past, the surface characteristics and the planar end-point of the planarized wafer surface have been detected by removing the semiconductor wafer from a polishing apparatus and physically examining it with techniques with which dimensional and planar characteristics can be ascertained. Typically, commercial instruments such as surface profilometers, ellipsometers, or quartz crystal oscillators are used for this purpose. If the semiconductor wafer being inspected does not meet specifications, it must be placed back into the polishing apparatus and further planarized. This is time-consuming and labor-intensive. In addition, if the inspection occurred too late; that is, after too much material has been removed from the wafer, the part becomes unusable and a reject. This adversely affected the product yield attainable with such processes and techniques.
It would therefore be desirable if a technique were available which permits one to control and terminate semiconductor device CMP processes effectively and efficiently. Some techniques proposed in the past involved utilizing sound generated during CMP for controlling the process and/or determining its end-point.
For example, U.S. Pat. No. 5,245,794 suggests to detect the CMP end-point during semiconductor wafer polishing by sensing acoustic waves which are generated by the rubbing contact between a polishing pad and a hard surface underlying a softer material that is being removed. Wave energy in the range of 35-100 Hz is sensed, converted into an audio signal, processed, and used to determine the end-point for the CMP after the signal has been sensed for a predetermined time.
U.S. Pat. No. 5,240,552 discloses to control a semiconductor wafer CMP by directing sound from an external source against the surface being polished and measuring the transit time of the acoustic waves reflected from the surface. From the latter, a desired characteristic, such as the amount of surface layer removed and/or remaining, can be calculated.
U.S. Pat. No. 5,439,551 discloses several CMP end-point detection techniques, including one that requires that a change in the sound waves emitted during polishing be detected and that polishing cease upon the detection of the change. A microphone-like, noncontact pick-up detects audible sound generated by the action of the polishing pad against the workpiece in the presence of a slurry. Although not specifically set forth in the '551 patent, it suggests that audible frequencies of sound are being measured because the patent discloses, amongst others, that the frequency of sound signals can be tailored. A still further approach for determining the CMP end-point is disclosed in U.S. Pat. No. 5,222,329. One aspect of this patent discloses to determine an interface end-point by detecting acoustic waves which develop a certain sound intensity versus frequency characteristic when the metal/underlayer interfaces are about to be reached in a CMP process. In other words, the signal amplitude in a certain frequency band is used to determine the end-point.
Another aspect of the '329 patent suggests to determine the end-point on the basis of a given material thickness by measuring the frequency of the acoustic waves generated by the CMP process and comparing the signals in a spectrum analyzer with known (or pre-established) frequency characteristics for the materials in question.
Although these prior art approaches provide certain improvements over earlier end-point detection techniques employing physical and/or optical measuring instruments, for example, they have their shortcomings. In some instances, the detected signals require complicated processing; in others, they require the storage of characteristic data for any given material before it can be measured, and all of them require relatively intricate, sensitive and therefore costly controls and instruments.